3D platforming can be done well, but on the whole 2D tends to work out better. Some of it is technical things like bad camera or just plain fugly graphics, but a lot of it is that having that third axis in a relatively precise game can make or break it very easily.

3D fighters can be very good, but tend to better for spectacle titles like the Soul series since they work towards making things look good as opposed to being better from a technical fighter perspective. SCIII is still the only fighting game I can claim to anything like competence because the gameplay is more loose and forgiving, where KoF, DoA, or Tekken give me fits since I tend to just pose at people and 2D fighters evolved beyond my ability to handle them long ago.

RPGs...well...they're complicated. Personally I feel that 2D exploration, or 2D-ish exploration (like say, Breath of Fire III and IV) works out better, since it's simpler to have things to do or interact with, but 3D 'real time' combat has a level of depth and energy that 2D real time usually doesn't. When it come to turn-based, sprites tend to work out a bit better, especially compared to early 3D's menagerie of Starfox LSD monstrosities.